3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network, UTRAN, is the radio access network of a UMTS and Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNodeB, in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
A currently popular vision of the future development of the communication in cellular networks comprises large numbers of small autonomous devices, which typically transmit and receive only small amounts of data infrequently, for instance once per week to once per minute. These devices are generally assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers for the purpose of configuration of and data receipt from said autonomous devices within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine, M2M, communication and the devices are denoted Machine Devices, MDs. The nomenclature used in 3GPP standardization for the communication is Machine Type Communication, MTC, whereas the devices are denoted MTC devices. As these devices are assumed to typically transmit rather seldom, their transmissions will in most cases be preceded by a Random Access, RA, procedure, which establishes the device's access to a network and reveals the device's identity to the network.
FIG. 1 schematically illustrates a cellular network 100 comprising a base station 110 and two wireless devices 120a, 120b, e.g. MTC devices. In a cell like the one disclosed in FIG. 1, wireless devices are located at different distances from the base station 110, wherein the channel characteristics vary due to different reasons e.g. distance to base station, disturbing radio sources or obstacles such as buildings.
An ongoing study item on low cost Machine Type Communication, MTC, in 3GPP Radio Access network, RAN 1 aims to enhance coverage with 20 dB coverage enhancements for low rate MTC devices. To achieve these coverage enhancements multiple channels will need to be improved. This disclosure aims at coverage enhancements in the random access procedure also referred to as RACH procedure. RACH stands for random access channel. A RACH is intrinsically a transport channel used by mobile phones and other wireless devices. However, the term RACH is often used as a general term referring to the random access procedure.
As an example, the random access procedure of a 3GPP Evolved Packet System, EPS, also known as a 3GPP Long Term Evolution/System Architecture Evolution, LTE/SAE, network is briefly described below.
In 3GPP Release 11, the Long Term Evolution, LTE, random access procedure is a four step procedure used for initial access when establishing a radio link, to re-establish a radio link after radio-link failure, to establish uplink synchronization or as a scheduling request if no dedicated scheduling-request resources have been configured on the Physical Uplink Control Channel, PUCCH.
3GPP Release 11 provides for a LTE random access procedure which is used in several situations: for initial access when establishing a radio link (moving from Radio Resource Control (RRC)_IDLE to RRC_CONNECTED state); to re-establish a radio link after radio-link failure; to establish uplink synchronization; or, as a scheduling request if no dedicated scheduling-request resources have been configured on the Physical Uplink Control Channel, PUCCH. The 3GPP Release 11 LTE random access procedure essentially comprises four basic steps which encompass a sequence of messages exchanged between the terminal and the eNodeB, as generally illustrated in FIG. 2. In FIG. 2, the four steps essentially correspond to the solid arrows, whereas the dotted arrows essentially correspond to control signaling for the solid arrow step which the dotted arrows precede. For example, the second step is the second arrow (dotted) and the third arrow (solid). The second arrow (dotted) tells the UE to listen to the third arrow corresponding to the second step. Further in the same way the fifth arrow tells the UE to listen to the fourth step in the RA-procedure corresponding to the last arrow. These basic four steps are briefly discussed below.
A first step in the random-access procedure comprises transmission of a random-access preamble on the Physical Random-Access Channel, PRACH. As part of the first step of the random-access procedure, the terminal randomly selects one preamble to transmit, out of one of the two subsets 301, 302 defined for contention-based access as illustrated in FIG. 3a. In LTE totally 64 preambles 300 are defined in each cell. Contention-based setup is used when there is a risk for collision of two UEs accessing the same resource. The subsets used for contention free setup 303 are used e.g. at handover, where there is no risk for collision.
Which subset to select the preamble from, is given by the amount of data the terminal would like to, and from a power perspective can, transmit on the Physical Uplink Shared Channel, PUSCH, in the third random access step. A time/frequency resource to be used for these transmissions is illustrated in FIG. 3b, which is understood by reading “4G-LTE/LTE Advanced for Mobile Broadband” by E. Dahlman et al, Academic Press, 2011. The time/frequency resource 310 to be used is given by the common PRACH configuration of the cell, which can be further limited by an optional, UE specific mask, which limits the available PRACH opportunities for the given UE. This is more thorough described in “3GPP TS 36.321 v.10.0.0. Medium Access Control (MAC) protocol specification” and “3GPP TS 36.331 v.10.3.0. Radio Resource Control (RRC) protocol specification”.
A second step of the random access procedure comprises the Random Access Response. In the Random Access Response the eNodeB transmits a message on the Physical Downlink Shared Channel, PDSCH, containing the index of the random-access preamble sequences the network detected and for which the response is valid; the timing correction calculated by the random-access preamble receiver; a scheduling grant; as well as a temporary identify, TC-RNTI, used for further communication between the UE and network. A UE which does not receive any Random Access Response in response to its initial random-access preamble transmission of step 1 above within a pre-defined time window, will consider the attempt failed, and will repeat the random access pre-amble transmission, possibly with higher transmit power, up to a number of maximum of four times, before considering the entire random-access procedure failed.
The third step of the random access procedure serves, e.g., to assign a unique identity to the UE within the cell, C-RNTI. In this third step, the UE transmits the necessary information to the eNodeB using the PUSCH resources assigned to the UE in the Random Access Response.
The fourth and last step of the random-access procedure comprises a downlink message for contention resolution. The message of this fourth step is also known as the RRC Connection Setup message. Based on the contention resolution message each terminal receiving the downlink message will compare the identity in the message with identity transmitted in the third step. Only a terminal which observes a match between the identity received in the fourth step and the identity transmitted as part of the first step will declare the random access procedure successful, otherwise the terminal will need to restart the random access procedure.
The UE power to use in the random access attempt is calculated according to a specified formula, known from “3GPP TS 36.213 v.10.6.0. Physical layer procedures”, reproduced as Expression 1 below, with parameters carried in the system information. If the UE does not receive a RandomAccessResponse in the second step of the procedure, the transmit power of the following PRACH transmission is increased by a parameter delta value up until limited by the UE maximum power:PPRACH=min{PCMAX,c(i),PREAMBLE_RECEIVED_TARGET_POWER+PLc}_[dBm]  Expression 1:
In Expression 1, PCMAX,c(i) is the configured UE transmitting power as defined in “3GPP TS 36.213 v.10.6.0. Physical layer procedures” for sub frame i of the primary cell and PLc is the downlink path loss estimate calculated in the UE for the primary cell.
As currently being discussed in 3GPP Coverage Enhancements TR 36.824, there are situations where a UE is unable to access the network due to Random Access Channel, RACH, coverage problems, e.g. the UE has Broadcast Control Channel, coverage and can thus measure on the cell and read the cell's system information, but the network cannot receive any random access preamble attempts from the UE because the UE is power/coverage limited, and hence the received signal in the network is too weak. This is the case, for example, for a user placed indoor served by a cell with high output power.
As an alternative starting from LTE Release 11, a UE can be configured to connect to multiple cells at once, i.e. one primary cell and one or several secondary cells and use so called carrier aggregation. In this case, the user equipment is also allowed to transmit RACH requests on the “secondary” cells, if the cells belong to different timing advance groups. However, if a device does not support carrier aggregation with multiple timing advance values random access is only allowed on the primary cell.
Hence, the above described random access procedure provides insufficient coverage. Repetition generally provides better coverage but provides limited opportunity for coherent combining and suffers from large power imbalances that affect orthogonality among sequences. Hence, power control such as applied for the LTE Release 8-10 formula has no benefits for users with significantly higher path loss than the path loss resulting in maximum power.
Hence, there is a need to provide a random access procedure which provides sufficient coverage and is suitable for low rate MTC devices.